The present invention relates to gas turbine engine operation, and more particularly, but not exclusively relates to the dynamic control of variable compressor elements during rapid transients.
Designers of axial flow compressors for gas turbine engines typically consider many issues associated with engine fluid flow, including stall and/or surge. A stall generally refers to a breakdown in fluid flow in only some of the stages in a multistage compressor and a surge generally refers to a complete breakdown of smooth fluid flow through the compressor and typically includes fluid flow reversal. The air flow and pressure ratio of the compressor at which a surge occurs tends to vary with compressor speed. A surge line represents the minimum stable air flow which can be obtained at any rotational speed of the subject compressor. Compressors are usually designed to have a surge safety margin between the air flow and pressure ratios at which they will normally be operated and the air flow and pressure ratios at which a surge will occur. This margin may be specified as a working line in terms of compressor pressure ratio versus fluid flow. The available surge margin for a given compressor tends to vary with a number of factors, including: production scatter associated with engine-to-engine variation; engine deterioration and fouling; transient excursions above the steady state working line; and instrument accuracy.
In some applications, control systems to provide this safety margin modulate the position of compressor mechanisms to vary its effective geometry, such as bleed off valves (BOVs) and variable inlet geometry vanes (VIGVs). For gas turbine engines that function as a prime mover of an electric power generator, one or more engine compressors may be constrained to rotate at an approximately constant speed that corresponds to the target frequency of Alternating Current (AC) electricity produced with the generator. Typically, a compressor subject to this constraint is not well-suited to control schemes that are used in other applications. Accordingly, there is a need for further advancements in this area of technology. Such advancements may not only find application to compressors operating at a constant speed, but also those operating with variable speed, and those that may or may not drive an electric power generator.